The present invention relates generally to antenna technology, and more particularly to a quadrifilar helix antenna assembly employing parallel LC resonant circuits to achieve dual band operation. This type of antenna can be applied, for example, in the context of satellite communications and navigation systems, such as have been undergoing development in the L band (for example INMARSAT, MSAT, PROSAT, NAVSTAR, GPS etc.) that can employ multiple frequencies. The relatively small size of the inventive antenna makes the antenna suitable for hand-held applications.
Many contemporary communications and navigation products have been developed that rely on earth-orbiting satellites to provide necessary communications and navigation signals. Examples of such products include satellite navigation systems, satellite tracking and locator systems (e.g., GPS), and communications systems (e.g., NAVSTAR) that rely on satellites to relay the communications signals from one station to another. Most of the antennas used in these products have been physically much too large to satisfy the requirements of emerging satellite phone applications. In order for these products to be operationally useful in hand-held equipment, the antennas they employ should be small (comparable or smaller in size than the receiver itself).
Several types of antennas are now used for hand-held receivers of satellite signals, such as GPS receivers. All are relatively compact and can receive right hand circularly polarized signals from any direction above the ground (e.g., hemispherical coverage, although gain along the ground or horizon can be low). The requirement for compact size has several performance benefits in addition to its obvious portability. It enables the pattern to have slowly varying gain and low frequency dispersion over most of the field of view. The latter is important to provide the desired location accuracy.
Quadrifilar helix antennas have become widely used for communication and navigation receivers that operate at UHF, L and S Band frequencies. A commonly used antenna on hand-held GPS receivers, both commercial and military, is a resonant quadrifilar helix with limited bandwidth. This is a four-arm helix that is less than one wavelength of the transmitted and received signals in length and therefore has a narrower bandwidth in comparison with a longer helix. The advantages of this type of antenna include its relatively compact size and its cardioid shaped pattern with excellent circular polarization coverage and low axial ratio over most of the field of view. Since it is a resonant antenna, its dimensions are chosen to provide optimal performance for one frequency band. C. C. Kilgus first described this type of antenna in xe2x80x9cResonant Quadrifilar Helixxe2x80x9d, IEEE Trans. Antennas and Propagation, Vol. AP-17, May 1969, pp 349-351.
Most of the patents and literature relating to GPS antennas are directed toward quadrifilar helix antennas"" use in civilian systems operating within one frequency band, L1. For the next generation of satellite-based communications and navigational equipment being developed, it will be necessary to simultaneously receive and/or transmit circularly polarized signals in multiple frequency bands (e.g., L1 and L2). Redundant reception of L1 and L2 signals will enhance reliability and be used to overcome ionospheric distortion. A description of several efforts in extending the bandwidth of quadrifilar helix antennas, particularly with the objective of accommodating separate frequency bands, can be found in a paper by J. M. Tranquilla and S. R. Best, xe2x80x9cA Study of the Quadrifilar Helix for Global Positioning System Applicationsxe2x80x9d, IEEE Trans. Antennas and Propagation, Vol. AP-38, Oct. 1990, pp 1545-1550. Each of the techniques described therein has some limitations with respect to size and/or performance:
Tapering the radius of a helix will increase its bandwidth. This is only appropriate, however, for longer helices that are not resonant (i.e., longer).
A xe2x80x9cpiggybackxe2x80x9d configuration of two helices, which are arranged coaxially with the low frequency helix below the high frequency helix, shows significant reduction of the gain and large phase variation for pattern of the low frequency helix at elevation angles below 45xc2x0.
Placing a high frequency helix coaxially inside a lower frequency helix can result in dual-band operation, but it does so with a degradation of the performance of both helices in all directions of the upper hemisphere.
The arms of two resonant quadrifilar helices can be interleaved. Because of the mutual coupling between the eight closely spaced arms, it is not practical to obtain low reflection loss and high radiation efficiency at two arbitrarily chosen frequencies. U.S. Pat. No. 5,828,348 to Tassoudji, et al., describes a low frequency band quadrifilar helix that is passively coupled to the high frequency helix. The input resistance of these antennas at the two resonance frequencies is low (10 to 15 ohms) and therefore requires transformers in each feed arm for impedance matching to the transmission lines of the feed network.
Thus, there remains a need for an antenna compact enough to be used in hand-held equipment, yet capable of operating in multiple satellite frequency bands. Such an antenna should also be capable of transmitting and/or receiving circularly polarized waveforms in different frequency bands, while providing a relatively high gain quasi-hemispherical radiation pattern over those bands.
The present invention provides a dual-band quadrifilar helix antenna capable of simultaneous operation in two frequency bands. In a preferred embodiment, the antenna is comprised of four radiating elements arranged helically to define a cylinder of constant radius, although other symmetric shapes are allowable. Each radiating element has an upper portion and a lower portion and a gap in between the upper and lower portions. Each upper portion of each radiating element has an electrically open end, and each lower portion of each radiating element has a feed point. The four feed points collectively receive 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 feed, or excite, signals (i.e., in phase quadrature) from a feed network. It is an advantage of the present invention that excitation of the antenna in both frequency bands may be effected through these common feed points.
A parallel LC (or xe2x80x9ctrapxe2x80x9d) circuit is disposed in the gap between, and electrically connects each upper and lower portion of each radiating element. The trap circuits each have a first impedance at a first frequency, and a second impedance greater than the first impedance, at a second frequency that is higher than the first frequency. This configuration enables dual frequency operation of the antenna at a lower first frequency being the resonant frequency corresponding to the entire length of each radiating element (including the trap), and at a second higher frequency being the resonant frequency corresponding to the length of the lower portion of each radiating element.
The radiating elements preferably have a left-hand twist over a single helical turn for receiving and/or transmitting right-hand circularly polarized signals. The signals intended to be received and/or transmitted have frequencies in the satellite frequency ranges, such as the L1 (1575 MHz) and L2 (1227 MHz) bands.
The antenna is preferably configured with a ground plane positioned beneath the four radiating elements. The ground plane operates to reflect the inherently backfire energy of the antenna.
In an alternative embodiment, the present invention provides a dual-band quadrifilar helix antenna which requires no ground plane. In this embodiment, the antenna is comprised of four radiating elements arranged helically with a right hand twist to define a cylinder of constant radius. Again, other symmetrical shapes are allowable. Each radiating element has an upper portion and a lower portion and a gap therebetween. Each lower portion of each radiating element has an electrically open end, and each upper portion has a feed point. The four feed points collectively receive feed signals in phase quadrature. Disposed within each gap between each upper portion and each lower portion of each radiating element lies a trap. Each trap is electrically connected to both the upper portion and the lower portion of one radiating element (or filar) at a position in that radiating element corresponding to the position at which the other traps are located in the other radiating elements. The trap circuit components are selected to provide the trap circuit with a first impedance at a first frequency, and a second impedance greater than the first impedance, at a second frequency that is higher than the first frequency. Because this embodiment requires excitation at the feed points at the xe2x80x9ctopxe2x80x9d of the antenna, a means for providing the excitation signals to the feed points that traverses the length of the antenna is required. In some embodiments, this requirement is satisfied by a hollow conduit that extends coaxially throughout the length of the antenna, through which the excitation signals may be carried (e.g., on coaxial cables).
In preferred embodiments, the radiating elements are etched onto a radiator portion of a microstrip substrate. The feed network may also be etched onto the microstrip substrate. For transmit operations, the feed network accepts input signals and performs necessary power division and phase control or adjustment to provide the signal phases necessary to feed the radiating elements of the antenna. For receive operations, the feed network accepts and combines the signals received from the radiating elements.